1. Field of the Invention
The present invention relates to an organic electroluminescent element (sometimes called a light-emitting element or an EL element) capable of emitting light by converting electric energy into light.
2. Description of the Related Art
An organic electroluminescent (EL) element is attracting attention as a promising display element because high-brightness light can be emitted with a low voltage. An important characteristic value of this organic electroluminescent element is an external quantum efficiency. The external quantum efficiency is calculated according to:external quantum efficiency φ=number of photons released from element/number of electrons injected into element
As this value is larger, the element is more advantageous in view of electric power consumption.
The external quantum efficiency of the organic electroluminescent element is determined according to:external quantum efficiency φ=internal quantum efficiency×light take-out efficiency
In the organic EL element utilizing fluorescence from an organic compound, the limit value of internal quantum efficiency is 25%, the light take-out efficiency is about 20% and therefore, the limit value of external quantum efficiency is considered to be about 5%.
As the means for more elevating the properties of a light-emitting element, a green light-emitting element utilizing light emission from an ortho-metalated iridium complex (Ir(ppy)3: tris-ortho-metalated complex of iridium(III) with 2-phenylpyridine) has been reported (see, for example, U.S. Patent Application Publication No. 2002/0034656). The phosphorescent element described therein is greatly enhanced in the emission efficiency of green and red lights as compared with the related-art singlet-based light-emitting elements, but improvements are demanded in view of durability.
Also, a light-emitting element using a metal complex for the host material has been proposed (see, for example, JP-A-2004-221065) as means for improving the driving durability, but more improvements are demanded in view of efficiency and durability.